Charles Ellicott Commentary Genesis 42:15

"hereby ye shall be proved: by the life of Pharaoh ye shall not go forth hence, except your youngest brother come hither." — Genesis 42:15 (ASV)

By the life of Pharaoh. —It was common in ancient times to swear by the king’s life (see 1 Samuel 17:55; 2 Samuel 14:19), and even by the life of Jehovah (2 Samuel 15:21; 2 Kings 2:2; 2 Kings 2:4; 2 Kings 2:6). It is only in the stricter morality of the Gospel that such oaths are forbidden (Matthew 5:33–37).